1. Field of the Invention
This invention relates to the recovery of petroleum from subterranean oil reservoirs. In particular, it relates to improved waterflooding operations involving the injection of a surfactant slug and drive fluid comprising concentrated brine.
2. Discussion of the Prior Art
In the recovery of oil from oil-bearing reservoirs, it usually is possible to recover only minor portions of the original oil in place by primary recovery methods which utilize only the natural forces present in the reservoir. A variety of supplemental recovery techniques has been employed in order to increase the recovery of oil from subterranean reservoirs. A widely used supplemental recovery technique is waterflooding, which involves the injection of aqueous media into the reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production well system through which the oil is recovered.
Interfacial tension between the injected waterflooding medium and the reservoir oil, the relative mobilities of the reservoir oil and injected media, and the wettability characteristics of the rock surfaces within the reservoir are factors which influence the amount of oil recovered by waterflooding. Thus, it has been proposed to add surfactants to the flood water in order to lower the oil-water interfacial tension and/or alter the wettability characteristics of the reservoir rock. Viscosifiers such as polymeric thickening agents may be added to all or part of the injected water in order to decrease the mobility ratio between the injected water and oil and improve the sweep efficiency of the waterflood.
Techniques involving the injection of an aqueous solution of brine-tolerant sulfonate surfactants have been developed for use under controlled conditions of salinity. Processes which involve the injection of aqueous surfactant solutions have been described in U.S. Pat. Nos. 3,508,612, 3,827,497, 3,890,239, 3,977,471 and 4,018,278, for instance. The surfactant slug may be followed by a thickened water slug which contains a viscosifier such as a water-soluble biopolymer in a graded concentration in order to provide a maximum viscosity greater than the effective viscosity of the flowing oil-water bank and a terminal viscosity near that of water. A driving fluid such as a field brine may be injected with or without the thickener to carry the process to conclusion.
By adding brine-tolerant surfactants to the injected fluids, recovery of the petroleum can be enhanced. However, employing adequate surfactant to substantially enhance the recovery of oil from the subterranean formation by the flooding water has not been generally economically feasible heretofore because the surfactants are adsorbed from the surfactant solution onto the rock surfaces of the subterranean formation. As a result of this adsorption of the surfactant, the concentration of the surfactant in the flooding water becomes less than that required to achieve enhanced recovery of the oil. Moreover, the adsorption, where the surfactant is a mixture, causes a chromatographic dispersion to separate components of the surfactant mixture on the basis of their relative sorptivity. Frequently, this dispersion destroys the efficacy of the surfactant mixture in lowering the interfacial tension between the flooding water and the oil being displaced within the formation.
Brine-tolerant surfactants are generally expensive chemical compositions, and less expensive sacrificial agents can be employed to prevent undue material losses. A surfactant composition has been found which overcomes most of the shortcomings of proprietary combinations. These combinations are not known in the prior art in waterflooding to recover oil from subterranean reservoirs.